I can pass the advice, someone gave me years ago: "Reading, reading, reading... and even more reading." Figure out, how others in your research area pharaphrase their sentences, how they argument, how they hide weak results.
I second the advice to look for a writing seminar or two, but in the longterm, you will only improve by constant reading the work of others. I started my writing career with "Writing scientific english" by Tim Skern. It's a small workbook, pointing out the basics for non-native english speakers and it helped me a lot. However, I think it will only help non-native speakers a lot.